Catalytic and Electrochemical Release of Solar Energy Stored in Strained Organic Compounds

我们设想，太阳能的转换和储存可以通过单分子体系中的分子内反应来整合。这种工艺将以最简单、最有效的方式将光伏和光化学方法的主要特点结合起来。这种分子内反应的典型例子是降冰片二烯(NBD)光化学转化为亚稳价异构体四环烷(QC)，这是一个单光子、单分子过程，其中键断裂和成键事件被最大限度地简化。从QC到NBD的反向反应释放高达100 kJ/mol的QC，使QC成为能量密度可与最先进的电池相媲美的太阳能燃料。该项目旨在产生基础知识，以释放应变有机分子中化学能量存储的全部潜力。与能量释放反应相关的三个主要挑战将被解决：(1)其催化触发：从对催化触发的环回反应及其不受欢迎的副反应，如脱氢和C-C键断裂的基本了解开始，我们的目标是开发在储存周期中具有更高选择性和更高可逆性的催化剂。(2)电化学触发：在对电化学触发的循环反转有基本了解的基础上，我们将通过设计电极和限制不良副反应和电极污染的电化学环境来提高储存循环的可逆性。(3)直接转化为电能：作为一项重大挑战，我们设想了一种“储能太阳能电池”，即将QC中储存的化学能直接转化为电能。为此，我们将设计具有合适的电子结构、化学结构和电化学稳定性的混合界面，以证明这一新概念的可行性。为了实现这些非常雄心勃勃的目标，Bachmann、Libua和Papp的团队已经合作，将他们在表面科学、原位光谱、电化学和材料科学方面的互补专业知识结合在一起。该项目团队将研究NBD/QC体系及其衍生物在超高真空、常压、固/液界面和(照片)电化学条件下的机理、动力学、能量学和稳定性。结合单晶研究、复杂模型催化剂和纳米结构材料等方面的研究，我们将对能量释放反应以及目前限制其可逆性的因素有一个详细的了解。然后，我们将利用这些知识来开发改进的催化和电化学存储系统，即分子、材料和方法的组合，以更好地控制、更有效和更可逆的方式实现“单光子、单分子”能量转换和存储。

We envision that solar energy conversion and storage could be integrated via intramolecular reactions in single molecule systems. Such processes would combine key features of photovoltaic and photochemical methods in the most simple and efficient manner. The prototypical example for such an intramolecular reaction is the photochemical conversion of norbornadiene (NBD) to its metastable valence isomer quadricyclane (QC), a single-photon, single-molecule process in which bond-breaking and bond-making events are simplified to the maximum. The reverse reaction from QC to NBD releases up to 100 kJ/mol, making QC a solar fuel with an energy density comparable to state-of-the-art batteries.This project aims at generating the fundamental knowledge to unleash the full potential of chemical energy storage in strained organic molecules. Three major challenges associated with the energy release reaction will be addressed: (1) Its catalytic triggering: Starting from a fundamental understanding of the catalytically triggered cycloreversion and its undesired side-reactions, such as dehydrogenation and C-C bond scission, we will aim at developing catalysts with enhanced selectivity and higher reversibility in the storage cycle. (2) Its electrochemical triggering: Based on a fundamental understanding of the electrochemically triggered cycloreversion, we will aim at improving the reversibility of the storage cycle by design of electrodes and electrochemical environments which limit undesired side-reactions and electrode fouling. (3) The direct conversion to electrical energy: As a grand challenge we envision an "energy-storing solar cell", i.e. the direct conversion of the chemical energy stored in QC to electrical energy. To this aim, we will design hybrid interfaces with appropriate electronic structure, chemical structure and electrochemical stability to demonstrate the feasibility of this new concept.To tackle these very ambitious goals, the groups of Bachmann, Libuda, and Papp have teamed up to combine their complementary expertise in surface science, in-situ spectroscopy, electrochemistry, and materials science. The project team will study the mechanism, kinetics, energetics and stability of the NBD/QC system and its derivatives in ultrahigh vacuum, at ambient pressure, at solid/liquid interfaces and under (photo)electrochemical conditions. Combining aspects of insight gained from single crystal studies, complex model catalysts, and nanostructured materials, we will obtain a detailed understanding of the energy release reaction and the factors that currently limit its reversibility. We will then use this knowledge to develop improved catalytic and electrochemical storage systems, i.e. combinations of molecules, materials, and methods enabling "single-photon, single-molecule" energy conversion and storage in a better controlled, more efficient, and more reversible fashion.